1. Technical Field of the Invention
The present invention relates to vanilloid analogues containing resiniferatoxin pharmacophores, pharmaceutical compositions comprising such analogues, and methods of using such analogues as vanilloid receptor agonists and potent analgesics.
2. Related Arts of the Invention
Capsaicin (CAP), which has the structure shown hereinafter, stimulates and then desensitizes sensory afferent C-fibers. The induced desensitization may have an application in arthritis, asthma, allergic responses including rhinitis, fever, pain, bladder hypersensitivity and the like.
Besides, the vanilloid receptor(VR)(or capsaicin receptor) is a specific neuronal membrane recognition site for CAP and related irritant compounds. It is expressed almost exclusively by primary sensory neurons involved in nociception and neurogenic inflammation. The receptor functions as a cation-selective ion channel with a preference for calcium, and its functional subtype, VR1, activated by both CAP and noxious heat has recently been cloned. Its desensitization caused by specific ligands has been recognized as a promising therapeutic approach to mitigate neuropathic pain and other pathological conditions in which neuropeptides released from primary sensory neurons play a crucial role.
Most exogenous VR agonists which are being developed or used as analgesics are structually related to CAP(i.e., Zostrix(trademark), Olvanil(trademark), SDZ-249482(trademark), and DA-5018(trademark)) and resiniferatoxin. Their structures include a common vanilloid ring which appears to be important for agonist activity. However, in a recent report, it was demonstrated that compounds lacking the vanilloid moiety, such as sesquiterpenoid unsaturated dialdehydes or triprenyl phenol, may also activate the receptor. Although receptor antagonists are fewer, several compounds such a capsazepine, which acts competitively at the CAP binding site, the channel blocker ruthenium red and capsazocaine have been reported.
Resiniferatoxin (RTX), a tricyclic diterpene isolated from Euphorbia resinifera, has been regarded as an ultrapotent CAP analogue. Indeed, the specific binding of RTX to the CAP binding site in dorsal root ganglia has been demonstrated with labeled [3H]RTX. RTX is being developed as an ultrapotent sensory neuron desensitizing agent for the treatment of urinary urge incontinence and the pain assoaciated with diabetic neuropathy. Recently, a totally enantiocontrolled synthesis and a conformational analysis of RTX have been reported. However, the pharmacophoric groups of RTX have not yet been clearly defined, although structure-activity studies suggest that the C20-homovanillic moiety, the C3-keto group, and the ortho-ester phenyl group on ring C are crucial structural elements responsible for the extremely high potency of RTX. The lower potency of CAP relative to RTX, on the other hand, may be rationalized by the lack of some of these critical pharmacophoric groups, especially C3-keto group.
U.S. Pat. No. 4,939,149 discloses a method for desensitizing a subject animal by administering a therapeutically effective desensitizing amount of RTX for desensitizing the animal to neurogenic inflammation, to chemically and thermally induced pain and to responses involving sensory afferent pathways sensitive to CAP.
Although a number of vanilloid agonist based on the structures of CAP and RTX have been reported as potential analgesics (e.g., U.S. Pat. No. 5,021,450 discloses homovanillyl diterpene derivatives such as 12-deoxyphorbol 13-phenylacetate 20-homovanillate and mezerein 20-homovanillate as mimics of RTX), these CAP-like analogues are limited by their intrinsic lower potency and narrow therapeutic index. RTX, on the other hand, is of limited availability from natural sources and is difficult to obtain synthetically due to its structural complexity.
The present inventors have made extensive researches to discover novel analgesic agents based on the vanilloid receptor, which has simpler structure than RTX or known RTX- or CAP-like analogues. As results thereof, we found that the new compounds having modifications on C20-homovanillic moiety, the C3-carbonyl, and the ortho-ester phenyl moiety as essential groups for recognition and binding showed potent vanilloid agonist activity in terms of the receptor binding assay and the CAP-activated single channel assay.
Thus, the present invention provides novel compounds represented by the following formula (I): 
wherein,
X is an oxygen or sulfuir atom;
A is xe2x80x94NHCH2xe2x80x94 or xe2x80x94CH2xe2x80x94;
R1 is a substituted or unsubstituted C1-4alkylaryl group, or the group of formula: R4COxe2x80x94 wherein R4 is an alkyl group having 1 to 18 carbon atoms, an alkenyl group having 2 to 18 carbon atoms, or substituted or unsubstituted aryl group having 6 to 10 carbon atoms;
R2 is a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, a haloalkyl group having 1 to 6 carbon atoms or a halogen atom;
R3 is a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an aminoalkyl, a diacid monoester or xcex1-alkyl acid; and
the asteric mark * indicates a chiral carbon atom, and their pharmaceutically acceptable salts.
The present invention also provides pharmaceutical compositions comprising the compound (I) as an active ingredient in an amount effective to alleviate pain, together with a pharmaceutically acceptable carrier.
The present invention also provides a use of the compounds as an active ingredient in medicines for treating pain or urinary incontinence.
The present invention still provides processes for preparing the compound (I).
The compounds of the present invention is represented by the following formula (I): 
wherein,
X is an oxygen or sulfur atom;
A is xe2x80x94NHxe2x80x94CH2xe2x80x94 or xe2x80x94CH2xe2x80x94;
R1 is a substituted or unsubstituted C1-4alkylaryl group, or the group of formula R4COxe2x80x94 wherein R4 is an alkyl group having 1 to 18 carbon atoms, an alkenyl group having 2 to 18 carbon atoms, or substituted or unsubstituted aryl group having 6 to 10 carbon atoms;
R2 is a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, a haloalkyl group having 1 to 6 carbon atoms or a halogen atom;
R3 is an aminoalkyl, a diacid monoester or xcex1-alkyl acid; and
the asteric mark * indicates a chiral carbon atom, and their pharmaceutically acceptable salts.
The preferred compounds may be represented by the following formula (I-a) and (I-b): 
wherein, R1, R2 and R3 have the same meanings as defined above.
In a preferred embodiment, the compounds have the general formula (I-a) or (I-b) wherein R1 is the group of formula R4COxe2x80x94 wherein R4 is an alkyl group having 1 to 18 carbon atoms; R2 is an alkyl group having 1 to 6 carbon atoms; and R3 is xe2x80x94(CH2)nNH2, xe2x80x94CO(xe2x80x94CH2)nCOOH or xe2x80x94(CH2)nCOOH, wherein n is an integer of 1 to 4. When R1 or R4 is a substituted aryl group, the substituents may be one or more lower alkyls having 1 to 4 carbon atoms or halogen atoms.
In a more preferred embodiment, the compounds have the general formula (I-a) or (I-b) wherein R1 is the group of formula R4COxe2x80x94 wherein R4 is an alkyl group having 1 to 6 carbon atoms; R2 is an alkyl group having 1 to 6 carbon atoms; and R3 is xe2x80x94CH2CH2NH2, xe2x80x94COCH2CH2COOH or xe2x80x94CH2COOH,
In all cases, the compound (I) may be in racemic mixture, or in R or S stereoisomer. And, the present invention encompasses the compound (I) in the form of their pharmaceutically acceptable salts.
The compounds of the invention may be chemically synthesized by the methods in the reaction schemes hereinafter, which are merely exemplary and in no way limit the invention. The reaction schemes show the steps for preparing the representative compounds of the present invention, and other compounds also may be produced by following the steps with appropriate modifications of reagents and starting materials, which are envisaged by those skilled in the art.
The syntheses of 3-acyloxy-2-benzylpropyl thiourea derivatives (7a-d) are outlined in Schemes 1 and 2. Monoalkylation of diethyl malonate with various benzyl halides, followed by LiAlH4-reduction, produced the corresponding diols (2a-d). Monoacetylation of 2, conversion of the remaining alcohol function to an azide, and deprotection of the acetyl group afforded the intermediate 3-azido-2-benzyl-1-propanols (5a-d). Acylation of 5a-d with various acyl halides produced the corresponding esters 6a-d, which following the reduction of the azide group were condensed with 4-methoxymethyloxy-3-methoxybenzylisothiocyanate and hydrolyzed to the final desired products 7a-d. The alternative ether analogues, represented by the 3-benzyloxy-2-benzylpropyl thiourea derivatives, 9a,c, were synthesized from 5a,c as shown in Scheme 2. Two chiral analogues of 7a-I were synthesized from (R)-3-acetoxy-2-benzyl-1-propanol ((R)-3a) obtained from the enantioselective enzymatic acetylation of racemic 2-benzyl-1,3-propanediol (Scheme 3) following the same procedure for the synthesis of 7a-I.
The synthesis of O-acetic acid analogues (19a,b) of above thiourea derivatives is represented in scheme 4. 4-Hydroxy-3-methoxy-benzonitrile (11) was protected by MOM (methylmethylether) group to give 12, whose cyano group was reduced to amine by LiAlH4 and then protected by CBZ (chlorobenzyl) group to 13. MOM group of 13 was deprotected and then the corresponding hydroxy group was alkylated by methyl bromoacetate to give 15. Methyl ester of 15 was hydrolyzed and then CBZ group was deprotected to give 17. Amine of 17 was condensed with isothiocyante 18a,b to give final desired products 19a,b, respectively.
The synthesis of O-succinic ester analogues (23a,b) of above thiourea derivatives is represented in scheme 5. Mono benzyl ester of succinic acid 20 was acylated with vanilloid 14 to give 21 whose benzyl groups are deprotected by hydrogenation to give 22. Amine of 22 was condensed with isothiocyanate 18a,b to give final desired products 23a,b, respectively.
The synthesis of O-aminoethyl analogues (28a,b) of amide derivatives is represented in scheme 6. Azido group of 24 was reduced to amine, which in situ was condensed with homovanillic pentafluoro ester to give 25. Hydroxy group of 25 was alkylated by dibromoethane to give 26 whose bromo group then was substituted by azido group to 27. Reduction of 27a,b afforded the final desired products 28a,b, respectively.
The synthesis of O-aminoethyl analogues (36a,b) of above thiourea derivatives is represented in scheme 7. Amine of 4-hydroxy-3-methoxybenzylamine 29 was protected by Boc group and then hydroxy was alkylated by dibromoethane to give 31. Bromo group of 31 was substituted by azido group and then N-Boc was deprotected to give 33. Amine of 33 was converted into the correspond isothiocyanate which was condensed with azide 6 to give thiourea 35. Azido group of 35a,b was reduced to afford final desired products 36a,b, respectively. 


The compounds of the present invention can be made into pharmaceutical compositions by combination with appropriate medical carriers or diluents. For example, the compounds of the present invention can be dissolved in oils, propylene glycol or other solvents commonly used to prepare injectable solutions. Suitable carriers include, but not limited thereto, physiological saline, polyethylene glycol, ethanol, vegetable oils and isopropyl myristate. For topical application, the compounds of the invention can be formulated as an ointment or cream.
The pharmaceutical compositions comprising the compound of the present invention may be applied for the following occasions:
to relieve pain caused by posttherpetic neuralgia, diabetic neuropathy, postrnastectomy pain syndrome, stump pain, reflex sympathetic dystrophy, trigeminal neuralgia, oral neuropathic pain, osteoarthritis, rheumatoid arthritis, fibromyalgia, Guillain-Barre syndrome, meralgia paraesthetica, burning mouth syndrome
to ameliorate pain such as intractable pain due to bilateral peripheral neuropathy
to relieve itch due to psoriasis, hemodyalisis, aquagenic pruritus, vulvar vestibulitis, notalgia paraesthetica, brachioradial prutitus, Lichen simplex chronicus
to treat cluster headache, vasomotor rhinitis or perenial allergic rhinitis in the form of intranasal drop
to treat bladder hypersensitivity or spinal detrusor hyperreflexia in the form of intravesical solution.
The compound of the present invention has potent analgesic and antiinflammatory activity, and the pharmaceutical composition of the present invention thus may be employed to alleviate or relieve acute, chronic or inflammatory pains, suppress inflammation, or treat urge incontinence.
The following formulation methods and excipients are merely exemplary and in no way limit the invention.
The compounds of the present invention in pharmaceutical dosage forms may be used in the form of their pharmaceutically acceptable salts, and also may be used alone or in appropriate association, as well as in combination with other pharmaceutically active compounds.
The compounds of the present invention may be formulated into preparations for injections by dissolving, suspending, or emulsifying them in aqueous solvents such as normal saline, Dextrose 5%, or non-aqueous solvent such as vegetable oil, synthetic aliphatic acid glycerides, esters of higher aliphatic acids or propylene glycol. The formulation may include conventional additives such as solubilizers, isotonic agents, suspending agents, emulsifying agents, stabilizers and preservatives.
The desirable dose of the compounds of the present invention varies depending on the condition and weight of the subject, severity, drug form, route and period of administration, and may be chosen by those skilled in the art. However, in order to obtain desirable effects, it is generally recommended to administer 0.0001-100 mg/kg, preferably 0.001-100 mg/kg by weight/day of the compounds of the present invention. The dose may be administered in single or divided into several times per day. In terms of composition, the compounds should be present between 0.0001 to 10% by weight, preferably 0.0001 to 1% by weight based on the total weight of the composition. The pharmaceutical composition of the present invention can be administered to a subject animal such as mammals (rat, mouse, domestic animals or human) via various routes. All modes of administration are contemplated, for example, administration can be made orally, rectally or by intravenous, intramuscular, subcutaneous, intrathecal, epidural or intracerebroventricular injection.